Electrode materials for magnesium batteries

ABSTRACT

A compound of formula A b′ Mg a M b X y  or A b′ Mg a M b (XO z ) y  for use as electrode material in a magnesium battery is disclosed, wherein A, M, X, b′, a, b, y, and z are defined herein.

RELATED APPLICATIONS

This application is a continuation-in-part of PCT Application No.PCT/US2011/37951, filed May 25, 2011, entitled “Electrode Materials forMagnesium Batteries”, which claims the benefit of U.S. ProvisionalPatent Application Ser. No. 61/348,068, filed May 25, 2010, which areincorporated herein by reference.

FIELD OF INVENTION

The subject matter generally relates to electrode materials for use inmagnesium batteries.

BACKGROUND

There is a continually increasing demand for devices capable of storingmore energy per unit volume (Wh/l) or energy per unit mass (Wh/kg) thantoday's premier rechargeable Li-ion batteries. One increasingly soughtafter route to meeting this demand is to utilize divalent magnesium(Mg²⁺), rather than the monovalent cation lithium (Li⁺) becausemagnesium enables nearly twice as much charge to be transferred, perweight or volume, than Li⁺ thus enabling high energy density.Furthermore the abundance of Mg metal and readily available compoundscontaining Mg can enable significant cost reduction relative to Li-ionbatteries.

SUMMARY OF THE INVENTION

Though the advantages to rechargeable Magnesium batteries are commonlyknown there has been no previous commercialization of this type ofbattery. The failure to commercialize Mg batteries is, at least in part,due to one central technical obstacle to enabling rechargeable Mgbatteries, i.e., the lack of suitable electrode materials capable ofallowing reversible Mg insertion and removal at an appreciable rate ofdischarge and charge. The electrode materials for Mg battery describedherein display low barrier to the diffusion of Mg while maintainingstability of the host structure in both the Mg-containing (magged) stateand the Mg-removed (de-magged) state while enabling useful reactionvoltage and capacity.

In one aspect, a compound of formula A_(b′)Mg_(a)M_(b)X_(y) for use aselectrode material in a magnesium battery is described,

wherein

A is one or more dopants selected from the group consisting of Al, Li,Na, K, Zn, Ag, Cu, and mixtures thereof;

M is one or more transition metals selected from the group consisting ofV, Cr, Mn, Fe, Co, Ni, Cu, Ag, Zr, and mixtures thereof.

X is one or more anions selected from the group consisting of O, S, Se,F, and mixtures thereof;

0≦b′≦2.9;

0≦a≦2.1;

0.5≦b≦2.9;

1.5≦y≦5.9; and

the compound has a layered structure or a spinel structure, wherein

-   -   the layered structure comprises close-packed anion X lattice,        layers of octahedrally-coordinated transition metal M, and        layers of fully or partially occupied magnesium sites, wherein        the layers of metal M and the layers of magnesium sites        alternate; and    -   the spinel structure comprises close-packed anion X lattice,        wherein the transition metal occupies the octahedral sites and        Mg occupies the tetrahedral sites;

provided that the compound is not layered Mg_(a)VS₂, spinel Mg_(a)Co₃O₄,layered Mg_(a)V₂O₅, rocksalt Mg_(a)MnO, spinel Mn₂O₄, spinelMg_(a)Mn₂O₄, spinel Mg_(a)Mn₃O₄, or layered Mg_(a)ZrS₂.

In some embodiments, b′ is 0 and the compound has a formula ofMg_(a)M_(b)X_(y).

In any of the preceding embodiments, b is about 1 and y is about 2.

In any of the preceding embodiments, b is about 2 and y is about 4.

In any of the preceding embodiments, M is one or more transitionalmetals selected from the group consisting of Cr, Mn, Ni, Co, andmixtures thereof and

X is one or more anions selected from the group consisting of O, S, F,and mixtures thereof.

In any of the preceding embodiments, the compound has a unit cell atomicarrangement isostructural with a layered material comprising primarilyMg layer dispersed between primarily transition metal layers.

In any of the preceding embodiments, the compound has a unit cell atomicarrangement isostructural to spinel unit cell and Mg occupies thetetrahedral site.

In any of the preceding embodiments, the compound has a magnesiumdiffusion barrier of less than 0.8 eV.

In any of the preceding embodiments, the compound is layered MgVO₃.

In any of the preceding embodiments, the compound is MgCr₂S₄ spinel.

In any of the preceding embodiments, 0.05≦b′≦3.9.

In another aspect, a compound of formula A_(b′)Mg_(a)M_(b)(XO_(z))_(y)for use as electrode material in a magnesium battery is described,wherein

A is one or more dopants selected from the group consisting of Al, Li,Na, K, Zn, Ag, Cu, and mixtures thereof;

M is one or more transition metals selected from the group consisting ofTi, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Sn, Sb, Bi, Ta, W, andmixtures thereof;

X is one or more anions selected from the group consisting of P, V, Si,B, C, As, S, N, and mixtures thereof;

0≦b′≦3.9;

0≦a≦3.1;

0.9≦b≦3.9;

0.9≦y≦3.9;

1.9≦z≦3.9; and

the compound has an olivine structure or a NASICON structure, or thecompound is isostructural with the LiVP₂O₇ structure or the vanadiumoxy-phosphate VO(PO₄) structure;

provided that the compound is not Mg_(a)MnSiO₄ or Mg_(0.5)Ti₂(PO₄)₃.

In any of the preceding embodiments, b′ is 0 the compound has a formulaof Mg_(a)M_(b)(XO_(z))_(y).

In any of the preceding embodiments, 0≦a≦2, b is about 1, y is about 1,and 3≦z≦3.9.

In any of the preceding embodiments, 0≦a≦3, b is about 2, y is about 3,and 3≦z≦3.9.

In any of the preceding embodiments, 0≦a≦1.53, b is about 0.5, y isabout 1, and 3≦z≦3.9.

In any of the preceding embodiments, the compound has a unit cell atomicarrangement isostructural with a monoclinic or rhombohedral NASICON unitcell.

In any of the preceding embodiments, X is P.

In any of the preceding embodiments, (XO_(z))_(y) is P₂O₇ and thecompound is isostructural with VP₂O₇.

In any of the preceding embodiments, the compound has a unit cell atomicarrangement isostructural with a beta-VOPO₄ unit cell.

In any of the preceding embodiments, the compound has a unit cell atomicarrangement isostructural with a cubic diphosphate TiP₂O₇ unit cell.

In any of the preceding embodiments, the compound has a magnesiumdiffusion barrier of less than 0.8 eV.

In any of the preceding embodiments, the compound is olivineMgFe₂(PO₄)₂.

In any of the preceding embodiments, the compound has the compound isNASICON MgFe₂(PO₄)₃ or MgV₂(PO₄)₃.

In any of the preceding embodiments, 0.05≦b′≦3.9.

In yet another aspect, a method of synthesizing a compound of any of thepreceding claims by solid state synthesis, co-precipitation, orcarbothermal reduction from magnesium containing precursor is described,wherein the magnesium containing precursor is one or more compoundsselected from the group consisting of MgO, Mg(OH)₂, MgCO₃, MgSO₄, MgS,MgF₂, MgHPO₄, Mg metal, and mixtures thereof.

In yet another aspect, a method of synthesizing a compound of any of theproceeding embodiments is described, comprising

using a precursor compound containing one or more metals selected fromthe group consisting of Cu, Zn, Ag, Na, K, Rb, Cd, Ca, Sr, Ba, andcombinations thereof and chemically or electrochemically extracting themetal and replacing the metal with Mg by chemical or electrochemicalinsertion.

In yet another aspect, a magnesium battery electrode is described,comprising a compound of any of the proceeding embodiments.

In yet another aspect, a magnesium battery electrode is described,comprising a compound of formula A_(b′)Mg_(a)M_(b)X_(y) for use aselectrode material, wherein

A is one or more dopants selected from the group consisting of Al, Li,Na, K, Zn, Ag, Cu, and mixtures thereof.

M is one or more transition metals selected from the group consisting ofV, Cr, Mn, Fe, Co, Ni, Cu, Ag, Zr, and mixtures thereof.

X is one or more anions selected from the group consisting of O, S, Se,F, and mixtures thereof

0≦b′≦2.9;

0≦a≦2.1;

0.5≦b≦2.9;

1.5≦y≦5.9; and

the compound has a layered structure or a spinel structure, wherein

-   -   the layered structure comprises close-packed anion X lattice,        layers of octahedrally-coordinated transition metal M, and        layers of fully or partially occupied magnesium sites, wherein        the layers of metal M and the layers of magnesium sites        alternate; and    -   the spinel structure comprises close-packed anion X lattice,        wherein the transition metal occupies the octahedral sites and        Mg occupies the tetrahedral sites;

provided that the compound is not layered VS₂, spinel Co₃O₄, layeredV₂O₅, rocksalt MnO, spinel Mn₂O₄, spinel Mn₃O₄, or layered ZrS₂.

In yet another aspect, a magnesium battery electrode is described,comprising a compound of formula A_(b′)Mg_(a)M_(b)(XO_(z))_(y) for useas electrode material in a magnesium battery, wherein

A is one or more dopants selected from the group consisting of Al, Li,Na, K, Zn, Ag, Cu, and mixtures thereof;

M is one or more transition metals selected from the group consisting ofTi, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Sn, Sb, Bi, Ta, W, andmixtures thereof;

X is one or more anions selected from the group consisting of P, V, Si,B, C, As, S, N, and mixtures thereof

0≦b′≦3.9;

0≦a≦3.1;

0.9≦b≦3.9;

0.9≦y≦3.9;

1.9≦z≦3.9; and

the compound has an olivine structure or a NASICON structure, or thecompound is isostructural with the LiVP₂O₇ structure or the vanadiumoxy-phosphate VO(PO₄) structure;

provided that the compound is not Mg_(a)MnSiO₄ or Mg_(0.5)Ti₂(PO₄)₃.

In any of the preceding embodiments, the compound has a layeredstructure.

In any of the preceding embodiments, the compound has a spinelstructure.

In any of the preceding embodiments, the electrode further comprises anelectronically conductive additive.

In any of the preceding embodiments, the conductive additive is carbonblack.

In any of the preceding embodiments, the electrode further comprises abinder.

In any of the preceding embodiments the binder is one or more compoundselected from the group consisting of polyvinylidene fluoride,polytetrafluoroethylene, and terpolymer or copolymer thereof.

In yet another aspect, a energy-storing device is described, comprising:

a first electrode comprising the compound of any of the proceedingembodiments; and

a second electrode comprising a magnesium metal, a magnesium alloy, orthe compound of any of the proceeding embodiments.

As used herein, two crystalline compounds are isostructural if they havethe same crystalline structure, but not necessarily the same celldimensions nor the same chemical composition, and with a comparablevariability in the atomic coordinates to that expected from the celldimensions and chemical composition.

As used herein, close-packed lattice is a term of art referring to adense arrangement of spheres in a lattice. It is well understood in theart that close-packing structure includes normal routine derivations.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an X-ray diffraction (XRD) pattern of MgMn₂O₄ spineldemonstrating chemical Mg extraction and electrochemical magnesiuminsertion according to one or more embodiments. This is the X-Raydiffraction spectra corresponding to a sample of MgMn₂O₄ spinel, whichwas synthesized, then immersed in acid to remove Mg. Following that thesample was placed in an electrochemical test cell to electrochemicallyreinsert Mg.

FIG. 2 is a voltage profile corresponding to MgMn₂O₄ spinel in aelectrochemical test cell demonstrating the higher activity observed byelectrochemically extracting Mg from the tetrahedral sites of MgMn₂O₄spinel at high voltage, than when attempting to insert additional Mginto the spinel to form a rock salt related compound at low voltage.

FIG. 3 depicts the X-ray diffraction spectra showing the progression ofcrystallinity of MgMn₂O₄ with various annealing temperatures (300°C.—top curve, 500° C.—middle curve, and 750° C.—bottom curve)

FIG. 4 shows the x-ray diffraction spectra showing the progression ofcrystallinity of MgNiMnO₄ with various annealing temperatures (330°C.—top curve, 500° C.—middle curve, and 750° C.—bottom curve).

FIG. 5 depicts the characterization of ZnCr₂S₄ spinel by powder X-raydiffraction.

FIG. 6 demonstrates the triphylite LiFePO₄ and the delithiated compoundFePO₄ characterized by powder XRD.

FIG. 7 shows the XRD comparison of lithium iron phosphate, LiFePO₄ (Topcurve) vs. de-lithiated LiFePO₄ (Bottom curve)

FIG. 8 demonstrates the synthesis of the polyanion compound vanadiumoxy-phosphate, VOPO₄, followed by chemical insertion of Mg by immersingthe dried powder in a solution of Butyl Magnesium in heptanes. Thesolution was stirred at about 30° C. for 6 days prior to removing thesample for X-Ray diffraction.

FIG. 9 illustrates the electrochemical Mg insertion into FePO₄ inmultiple Mg-containing electrolytes.

FIG. 10 shows an XRD characterization of the layered material CuCrS₂,indexed to PDF reference card 23-0952 with an excellent figure of merit.

FIG. 11 shows an XRD characterization of the layered material FeOCl,indexed to PDF reference card 39-0612 with a good figure of merit.

FIG. 12 shows XRD comparisons of experimental and theoreticalLi_(x)V₁₂O₂₉ (x=0-3.5) Patterns from top to bottom: theoretical V₁₈O₄₄,theoretical Li₃V₁₂O₂₉, theoretical Li₂V₁₂O₂₉, theoretical LiV₁₂O₂₉,theoretical V₁₂O₂₉, Actual sample which was indexed to PDF referencecard 44-0379 corresponding to Na_(0.6)V₁₂O₂₉.

DETAILED DESCRIPTION

The materials and classes of materials, described herein, are promisingfor use as magnesium insertion materials in magnesium-ion batteries. Therate of magnesium insertion into these materials is comparable to orbetter than the rate of the magnesium insertion into Chevrel-phasecompounds, and a battery with a magnesium anode and one of thesematerials as cathode has significantly higher theoretical energy densityand specific energy than a similar battery with a Chevrel-phase cathode.Chevrel compounds are series of ternary molybdenum chalcogenidecompounds first reported by R. Chevrel, M. Sergent, and J. Prigent in1971. The Chevrel compounds have the general formula M_(x)Mo₆X₈, where Mrepresents any one of a number of metallic elements throughout theperiodic table; x has values between 1 and 4, depending on the Melement; and X is a chalcogen (sulfur, selenium or tellurium).

Materials as active materials in the electrodes of rechargeableMagnesium (Mg) batteries are described. These materials demonstrate highMg mobility through the host crystal structure when the material is inboth the charged and discharged state thus enabling transfer of chargeto occur at useful rates during charge and discharge. In certainembodiments, the magnesium material has a magnesium diffusion barrier ofless than 0.8 eV. The low diffusion barrier of the materials asdescribed herein enables the material to be used as electrode activematerial in a magnesium battery. Additionally, the materials asdescribed herein exhibit useful reaction voltage, high theoreticalspecific capacity, and stability during the electrochemical reaction.

Applicants have surprisingly discovered that compounds or materialshaving a magnesium diffusion barrier of less than 0.8 eV result in highrates of Mg-insertion into the compounds and Mg-extraction out of thecompounds, which enables the compounds to be used in a magnesiumbattery. In some embodiments, the compounds as described herein has amagnesium diffusion barrier of less than 0.8 eV. In some embodiments,the compounds as described herein has a magnesium diffusion barrier ofless than 0.7 eV. In some embodiments, the compounds as described hereinhas a magnesium diffusion barrier of less than 0.6 eV. In someembodiments, the compounds as described herein has a magnesium diffusionbarrier of less than 0.5 eV. In some embodiments, the compounds asdescribed herein has a magnesium diffusion barrier of less than 0.4 eV.In some embodiments, the compounds as described herein has a magnesiumdiffusion barrier of less than 0.3 eV. In some embodiments, thecompounds as described herein has a magnesium diffusion barrier of lessthan 0.2 eV. In some embodiments, the compounds as described herein hasa magnesium diffusion barrier of less than 0.1 eV. In some embodiments,the compounds as described herein has a magnesium diffusion barrier ofmore than 50 meV and less than 0.8 eV. In some embodiments, thecompounds as described herein has a magnesium diffusion barrier of morethan 100 meV and less than 0.7 eV. In some embodiments, the compounds asdescribed herein has a magnesium diffusion barrier of more than 150 meVand less than 0.6 eV. In some embodiments, the compounds as describedherein has a magnesium diffusion barrier of more than 200 meV and lessthan 0.5 eV. The low magnesium diffusion barrier of the compounds asdescribed herein allows efficient reversible Mg insertion and removal atan appreciable rate of discharge and charge and enables the materials tobe used as electroactive materials for the magnesium electrodes.Furthermore, materials with high Mg mobility barriers would be excluded,based on expected poor Mg mobility and therefore very low ratecapability. These criteria based on computations of Mg barriers providea powerful means of identifying materials (known and unknown) with goodMg mobility and hence potential application as Mg electrode materials.

In one aspect, a compound of formula A_(b′)Mg_(a)M_(b)X_(y) for use aselectrode material in a magnesium battery is described, wherein

A is one or more dopants selected from the group consisting of Al, Li,Na, K, Zn, Ag, Cu, and mixtures thereof;

M is one or more transition metals selected from the group consisting ofV, Cr, Mn, Fe, Co, Ni, Cu, Ag, Zr, and mixtures thereof;

X is one or more anions selected from the group consisting of O, S, Se,F, and mixtures thereof;

0≦b′≦2.9;

0≦a≦2.1;

0.5≦b≦2.9;

1.5≦y≦5.9; and

the compound has a layered structure or a spinel structure, wherein

-   -   the layered structure comprises close-packed anion X lattice,        layers of octahedrally-coordinated transition metal M, and        layers of fully or partially occupied magnesium sites, wherein        the layers of metal M and the layers of magnesium sites        alternate; and    -   the spinel structure comprises close-packed anion X lattice,        wherein the transition metal occupies the octahedral sites and        Mg occupies the tetrahedral sites;

not layered Mg_(a)VS₂, spinel Mg_(a)Co₃O₄, layered Mg_(a)V₂O₅, rocksaltMg_(a)MnO, spinel Mn₂O₄, spinel Mg_(a)Mn₂O₄, spinel Mg_(a)Mn₃O₄, orlayered Mg_(a)ZrS₂.

In some embodiments, the compound is not layered A_(b′)Mg_(a)VS₂, spinelA_(b′)Mg_(a)Co₃O₄, layered A_(b′)Mg_(a)V₂O₅, rocksalt A_(b′)Mg_(a)MnO,spinel A_(b′)Mg_(a)Mn₂O₄, spinel A_(b′)Mg_(a)Mn₃O₄, or layeredA_(b′)Mg_(a)ZrS₂. In some embodiments, the compound is not layered VS₂,spinel Co₃O₄, layered V₂O₅, rocksalt MnO, spinel Mn₂O₄, spinel Mn₂O₄,spinel Mn₃O₄, or layered ZrS₂.

In some embodiments, b′ is 0 and the compound has a formula ofMg_(a)M_(b)X_(y). In some embodiments, b′ is not 0 and the dopant Apartially substitute for the transition metals to enhance theperformance or cost of the electrode material. Batteries containingmagnesium anodes and cathodes comprising of these materials have alsohigher theoretical energy density and specific energy than currentcommercial lithium-ion batteries, specified by a carbonaceous insertionanode.

In certain embodiments, the material is a layered compound having thegeneral formula Mg_(a)M_(b)X_(y), wherein “M” is a metal cation, ormixture of metal cations and “X” is an anion or mixture of anions. Insome embodiments, X is oxygen (O), sulfur (S), selenium (Se) or fluoride(F), or mixtures thereof. The structures can have a close-packed latticeof O, S, Se, or F, with layers of octahedrally-coordinated metals thatare capable of being oxidized during Mg extraction (for example,selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, Ag, ormixtures thereof) alternating with layers of fully or partially occupiedmagnesium sites. In certain embodiments, M is one or more transitionalmetals selected from the group consisting of Cr, Mn, Ni, Co, andmixtures thereof; and X is one or more anions selected from the groupconsisting of O, S, F, and mixtures thereof. In other embodiments, M isone or more transitional metals selected from the group consisting of V,Cr, Mn, Fe, Ni, Co, and mixtures thereof; and X is one or more anionsselected from the group consisting of O, S, F, and mixtures thereof.

In some embodiments, b′ is 0. In some embodiments, the compound is in anoxidized state and a is about 0. In some embodiments, the compound is ina reduced state and a is about 2. In some embodiments, b is about 1 andy is about 2. In other embodiments, b is about 2 and y is about 4.

In certain embodiments, b′ is in the range of 0.05≦b′≦2.9. In certainembodiments, b′ is in the range of 0.05≦b′≦2.0. In certain embodiments,b′ is in the range of 0.05≦b′≦1.5. In certain embodiments, b′ is in therange of 0.05≦b′≦1.0. In certain embodiments, b′ is in the range of0.1≦b′≦2.0. In certain embodiments, b′ is in the range of 0.2≦b′≦1.5. Incertain embodiments, a is in the range of 0≦a≦2. In certain embodiments,a is in the range of 0.5≦a≦1.5. In certain embodiments, a is in therange of 0.75≦a≦1.25. In certain embodiments, b is in the range of0.5≦b≦2. In certain embodiments, b is in the range of 0.75≦b≦1.5. Incertain embodiments, b is in the range of 0.75≦b≦1.0. In certainembodiments, a is in the range of 0.75≦a≦1.25. In certain embodiments, yis in the range of 1.5≦y≦5.0. In certain embodiments, y is in the rangeof 2.0≦y≦4.5. In certain embodiments, y is in the range of 2.5≦y≦3.5. Incertain embodiments, y is in the range of 3≦y≦3.5. All ranges of a, b,b′, and y can be combined with any of the recited ranges for a, b, b′,and y.

In one or more embodiments, the material includes layered transitionmetal oxides, sulfides, and selenides, with layers ofoctahedrally-coordinated transition metals alternating with layers offully or partially occupied magnesium sites. In particular embodiments,the layered compound include oxides containing transition metals such asV, Cr, Ni, Mn, Co, or mixtures thereof on the transition metal site.Examples of compositions that are able to insert nearly one magnesiumion per two transition metal ions include CoMn₂O₆ and CrS₂. In otherembodiments, the material includes sulfides and selenides containing V,Mn, or Cr as the transition metals. These sulfide and selenide materialsprovide lower voltage (˜0.25 V to ˜2.25 V vs. Mg/Mg²) and may also beuseful in magnesium insertion anodes.

In some embodiments, the compound described herein has a unit cellatomic arrangement isostructural with a layered material comprisingprimarily Mg layer dispersed between primarily transition metal layers.In some embodiments, the compound described herein has a unit cellatomic arrangement isostructural to spinel unit cell and Mg occupies thetetrahedral site. In some specific embodiments, the compound is layeredMgVO₃. In some specific embodiments, the compound is MgCr₂S₄ spinel.

In some embodiments, the compound described herein can be synthesized bycationic exchange of magnesium for the lattice cation in compounds suchas AMX₂ wherein A is preferably Na, K, Cu, Ag and X═O, S, Se, or F.Possible starting compositions identified for this purpose includeNaCrS₂, NaVS₂, CuFeO₂, CuCoO₂, NaCoO₂, CuNi_(0.33)V_(0.67)O₂, KCrS₂,AgNiO₂, AgCrO₂, KCrO₂, NaCrSe₂, NaVSe₂. In some embodiments, the cation‘A’ is electrochemically or chemically removed or exchanged. In theseembodiments, Li is used (as described in U.S. Pat. No. 6,426,164, whichdescribes use of LiCoO₂ or LiNiO₂). Lithium, due to its size, can alterthe lattice parameter of some ternary layered transition metal oxides ina fashion that is less favorable for migration of Mg into the hoststructure, than the A site cations identified above. Furthermore somecombinations of Li/M allow for greater degree of disorder of thetransition metal hopping into the Li layer, which can also inhibit Mgmigration. The A-site cations proposed herein when A=Na, K, Cu, or Agcan mitigate this disorder, promoting facile migration of Mg into andout of the host structure.

In some embodiments, magnesium vanadium oxides include MgV₂O₅ and MgVO₃.Though Mg insertion into V₂O₅ has been previously examined, no previousworks exists in which MgV₂O₅ is directly synthesized and Mg iselectrochemically removed. The as-synthesized MgV₂O₅ structure isdifferent than the as-synthesized V₂O₅ structure as there is a differentstacking of V₂O₅ layers between them. This enables a difference in Mgdiffusion within the two forms of MgV₂O₅. The direct synthesis and thenelectrochemical removal of Mg is preferred on this basis. In someembodiments, the ternary vanadium oxide, MgVO₃, is used as an electrodematerial in an Mg battery.

In some embodiments, the compound is spinel MgAl₂O₄. In otherembodiments, the compound has a structure isostructural with spinel(MgAl₂O₄). Spinels are a class of compounds that crystallize in thecubic (isometric) crystal system, with the oxide anions arranged in acubic close-packed lattice and the cations A and B occupying some or allof the octahedral and tetrahedral sites in the lattice. Some spinelsundergo a cubic to tetragonal distortion of the lattice when formingwith Mg in the tetrahedral sites. The electroactive magnesium spinelcompound can have the general formula Mg_(a)M_(b)X_(y), wherein “M” is ametal cation or mixture of metal cations (for example, selected from thegroup consisting of V, Cr, Mn, Fe, Co, Ni, Cu, Ag, Zr, or mixturesthereof) and “X” is an anion or mixture of anions, most often oxygen(O), sulfur (S), selenium (Se) or fluoride (F). The transition metalsoccupy the octahedral sites and magnesium occupies the tetrahedral sitesfor the preferred materials described herein. Spinel materials in whichall of the tetrahedral sites are occupied by a metal cation other thanMg (e.g. Co₃O₄) are excluded here as they exhibit high barriers to Mgdiffusion. For example Co in Co₃O₄ spinel occupies both tetrahedral andoctahedral sites, so upon Mg insertion the structure becomes akin torock salt structure, rather than spinel. In some embodiments, otherelements, such as Al, Li, Zn, and Mg, may be partially substituted forthe transition metals to enhance the performance or cost of theelectrode material. The magnesium ion may be partially or completelyreversibly extracted from the material electrochemically yieldingspecific capacity >150 mAh/g. Batteries containing magnesium anodes andcathodes comprising these materials are capable of higher energy densityand specific energy than commercial lithium-ion batteries.

In other embodiments, the compound is spinel MgMn₂O₄. In theseembodiments, the compound has a tetragonal form. In other embodiments, acubic modification to the tetragonal form can occur, which increases theMg to Mn ratio. In some specific embodiments, the Mg to Mn ratio is 2.In still other embodiments, the compound described herein is a spinelMgMn₂O₄, spinel. In still other embodiments, the compound describedherein includes a spinel having the following transition metals: Ni,Ni/Mn, Co, Ni/Mn/Co, Fe, Cr, Cr/Mn, V, or V/Cr.

Previous work on reactions of Mg with manganese oxides have beenpublished as well as studies in which Li⁺ was removed from LiMn₂O₄ andthen replaced (chemically or electrochemically) with Mg. In Kurihawa et.al, Chemistry Letters Vol. 37, 376-377 (2008), MgMn₂O₄ was prepared fromone of the example sets of precursors as described herein (i.e. from MgOand Mn₂O₃), however the synthesis of MgMn₂O₄ was included by“atmospheric pressure microwave discharge using CF pieces”, rather thanby the solid-state synthesis, carbothermal reduction, orco-precipitation methods as described herein. Furthermore, the highestdiffusion of Mg within the spinel structure is obtained by extracting Mgfrom the material according to the reaction:

MgMn₂O₄→Mg_(1−x)Mn₂O₄+xMg²⁺+2xe−  [1]

Hence the more favorable Mg diffusion will enable a greater degree of Mgextraction, and closer to the theoretical limit of capacity (270 mAh/g)at ˜2.9 V vs. Mg/Mg²⁺ according to the modeling studies describedherein. In contrast, Kurihawa et. al. only explores Mg insertion intoMgMn₂O₄ between 2.0 and ˜0.5 V vs. Mg/Mg²⁺, which accordinglycorresponds to the following reaction:

MgMn₂O₄ +xMg²⁺+2xe−→Mg_(1+x)Mn₂O₄  [2]

Experimentally validation of a 3V magnesium cathode material isdemonstrated in FIGS. 1 and 2. FIG. 1 is a series of XRD spectra whichshow the chemical Mg removal and electrochemical reinsertion of Mg fromthe MgMn₂O₄ phase. FIG. 2 shows the initial charge/discharge testing oftetragonal spinel MgMn₂O₄. From these data it is clear thatelectrochemical removal of magnesium from the spinel (reaction [1])initiates at about 2.7 V. There is a change in slope occurring at ˜1.7V, which is the calculated voltage for magnesium insertion into MgMn₂O₄.The calculations indicate that this phase exhibits significantly higherdiffusion barriers during the low voltage reaction [2] thus explainingthe minimal capacity observed within that region by us and Kurihawa et.al. experiments. Consequently, the spinel materials described withinthis application target those capable of the Mg extraction reactiondescribed by equation [1].

In certain embodiments, the spinel compounds are oxides containing oneor more metals selected from the group consisting of Cr, V, Fe, Co, Ni,Mn, and mixtures thereof, as the transition metals. Non-limitingexamples include MgCr₂O₄, MgV₂O₄, MgFe₂O₄, MgMg₅V₄O₁₂, MgCo₂O₄, MgMn₂O₄,MgNi₂O₄, MgCrVO₄, MgCrCoO₄, MgNiMnO₄, MgCoMnO₄, MgMnVO₄, MgFeNiO₄,MgCrNiO₄, MgNiVO₄, MgCoVO₄, Mg₃FeV₅O₁₂, Mg₃MnV₅O₁₂, Mg₂CrV₃O₈,Mg₂VCr₃O₈, Mg₂FeV₃O₈, Mg₂VFe₃O₈, Mg₃CrV₅O₁₂, Mg₃Fe₂V₄O₁₂, Mg₃Fe₁V₅O₁₂,Mg₃V₁Mn₅O₁₂, Mg₃Cr₂V₄O₁₂, Mg₃V₂Fe₄O₁₂, Mg₃Cr₁V₅O₁₂, Mg₃Cr₄V₂O₁₂,MgFeVO₄, MgNiVO₄, Mg₃Ni₂V₄O₁₂, Mg₂MnV₃O₈, Mg₃Co₂V₄O₁₂, Mg₂NiMn₃O₈,Mg₃Ni₁Mn₅O₁₂, Mg₃Ni₂Mn₄O₁₂, Mg₂NiFe₃O₈, Mg₃Ni₁Fe₅O₁₂, Mg₃Ni₂Fe₄O₁₂,Mg₃Ni₁Cr₅O₁₂, Mg₂NiCr₃O₈, Mg₃Ni₂Cr₄O₁₂. In other embodiments, the spinelcompounds includes spinel compounds of sulfides and selenides containingone or more metals selected from the group consisting of Zr, V, Mn, Cr,and mixtures thereof. Non-limiting examples include MgZr₂S₄, MgZr₂Se₄,MgV₂S₄, MgV₂Se₄, MgCr₂S₄, MgCr₂Se₄, MgMn₂S₄, MgMn₂Se₄, MgCrVS₄,MgCrVSe₄. In some embodiments, these materials are used as cathodeactive materials. In other embodiments, these materials provide lowervoltage (˜0.25 V to −2.5 V vs. Mg/Mg²⁺) and are used as magnesiuminsertion anode active materials.

Spinel materials may be synthesized through a variety of methods. Insome embodiments, spinel materials may be synthesized by solid statesynthesis in which MgO or Mg(OH)₂ is reacted with a manganese oxide suchas Mn₂O₃ to create MgMn₂O₄. Another route of synthesis involvesco-precipitation of the above reactions from solution (e.g. aqueous) inorder to obtain a finely divided mixture for subsequent heating. Inother embodiments, these materials are synthesized by two-step reactionwherein a non-Mg divalent metal is first reacted with a binary compoundof the transition metal to form an intermediate compound with the spinelstructure and the preferred ordering of the non-Mg divalent metal in thetetrahedral sites. The second step consists of removal of theplaceholder non-Mg divalent cation and magnesium insertion. For exampleZnC₂O₄.2H₂O+V₂O₅+C->ZnV₂O₄+2CO₂+CO+2H₂O followed by Zn extraction, andthen Mg insertion to form MgV₂O₄. In some embodiments, compounds thatcan be utilized as intermediates for the two-step reaction includeCuMn₂O₄, CuFe₂O₄, ZnCr₂O₄, ZnFe₂O₄.

In some embodiments, MgCr₂S₄ spinel and sulfide spinels including Zr, V,Mn, and Cr, or mixtures thereof are synthesized in a single step byreacting MgS and a binary transition metal sulfide under conditions ofsolid state synthesis, e.g., MgS+Cr₂S₃->MgCr₂S₄. In other embodiments,these materials are synthesized by two-step reaction wherein a non-Mgdivalent metal sulfide is first reacted with a binary transition metalsulfide under solid state conditions, to form an intermediate compoundwith the spinel structure and the preferred ordering in which the non-Mgdivalent metal occupies the tetrahedral site. The second step consistsof removal of the placeholder non-Mg divalent cation and subsequentmagnesium insertion. In some specific embodiments, the solid statereaction of CuS+Cr₂S₃->CuCr₂S₄ is followed by chemical orelectrochemical Cu extraction, and then Mg insertion to form MgCr₂S₄.Non-limiting examples of intermediate compounds for the two-stepreaction include: CuV₂S₄, ZnCr₂S₄, CuCo₂S₄, CuZr₂S₄, CuCr₂S₄.

In another aspect, a compound of formula A_(b′)Mg_(a)M_(b)(XO_(z))_(y)for use as electrode material in a magnesium battery is described,wherein

A is one or more dopants selected from the group consisting of Al, Li,Na, K, Zn, Ag, Cu, and mixtures thereof;

M is one or more transition metals selected from the group consisting ofTi, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Sn, Sb, Bi, Ta, W, andmixtures thereof;

X is one or more anions selected from the group consisting of P, V, Si,B, C, As, S, N, and mixtures thereof;

0.9≦b′≦3.9;

0≦a≦3.1;

0.9≦b≦3.9;

0.9≦y≦3.9;

1.9≦z≦3.9; and

the compound has an olivine structure or a NASICON structure, or thecompound is isostructural with the LiVP₂O₇ structure or the vanadiumoxy-phosphate VO(PO₄) structure;

provided that the compound is not Mg_(a)MnSiO₄ or Mg_(0.5)Ti₂(PO₄)₃.

In some embodiments, “M” is a metal cation such as Ti, V, Cr, Mn, Fe,Co, Ni, Cu, Zr, Nb, Mo, Sn, Sb, Bi, Ta, or W and “X” in a non-metalcation such as carbon (C), boron (B), phosphorous (P), silicon (Si),sulfur (S), nitrogen (N), arsenic (As). In some embodiments, thecompound comprises a polyanion “XO_(z)”. In some embodiments, thecompound has an olivine structure. In other embodiments, the compoundhas a NASICON structure. The materials can have an olivine-typestructure, a NASICON type structure, diphosphate-type structure, orvanadium oxy-phosphate type structure.

In some embodiments, b′ is 0 and the compound ahs a formula ofMg_(a)M_(b)(XO_(z))_(y).

In some embodiments, the compound is in an oxidized state and a is about0. In some embodiments, the compound is in a reduced state and a isabout 2. In some embodiments, b is about 1 and y is about 2. In otherembodiments, b is about 2 and y is about 3.9.

In certain embodiments, b′ is in the range of 0.05≦b′≦2.9. In certainembodiments, b′ is in the range of 0.05≦b′≦2.0. In certain embodiments,b′ is in the range of 0.05≦b′≦1.5. In certain embodiments, b′ is in therange of 0.05≦b′≦1.0. In certain embodiments, b′ is in the range of0.1≦b′≦2.0. In certain embodiments, b′ is in the range of 0.2≦b′≦1.5. Incertain embodiments, a is in the range of 0≦a≦2. In certain embodiments,a is in the range of 0.5≦a≦1.5. In certain embodiments, a is in therange of 0.75≦a≦1.25. In certain embodiments, b is in the range of0.5≦b≦2. In certain embodiments, b is in the range of 0.75≦b≦1.5. Incertain embodiments, b is in the range of 0.75≦b≦1.0. In certainembodiments, a is in the range of 0.75≦a≦1.25. In certain embodiments, yis in the range of 1.0≦y≦3.9. In certain embodiments, y is in the rangeof 1.5≦y≦3.5. In certain embodiments, y is in the range of 2.0≦y≦3.0. Incertain embodiments, y is in the range of 3≦y≦3.5. In certainembodiments, y is in the range of 1.0≦y≦2.0. In certain embodiments, zis in the range of 2.0≦y≦3.9. In certain embodiments, y is in the rangeof 2.5≦z≦3.5. In certain embodiments, y is in the range of 2.5≦z≦3.0. Incertain embodiments, y is in the range of 3≦z≦3.5. All ranges of a, b,b′, z, and y can be combined with any of the recited ranges for a, b,b′, z, and y.

In some embodiments, 0≦a≦2, b is about 1, y is about 1, and 3≦z≦3.9. Inother embodiments, 0≦a≦3, b is about 2, y is about 3, and 3≦z≦3.9. Instill other embodiments, 0≦a≦1.53, b is about 0.5, y is about 1, and3≦z≦3.9.

In some embodiments, the compound described herein is isostructural withan olivine LiFePO₄, in which the “Li” site is fully or partiallyoccupied by magnesium, the “Fe” site is occupied by a transition metal,and the “P” site is occupied by a cation. Batteries containing magnesiumanodes and cathodes comprising of these materials should have energydensity and specific energy similar to or greater than commerciallithium-ion batteries.

In some embodiments, compounds with specific olivine, NASICON,diphosphate-type structures, or vanadium oxy-phosphate type structuressuitable for use as electroactive materials in magnesium batteries aredisclosed.

In certain embodiments, the magnesium battery materials or compounds isisostructural with an olivine LiFePO₄, which contains Mn, Fe, Co, Ni,Cr, Cu, or mixtures thereof on the “Fe” site and P, As, Si, V, or S onthe “P” site. Non-limiting examples of such material or compound includeMgFe₂(PO₄)₂, MgCr₂(PO₄)₂, MgMn₂(PO₄)₂, Mg₂Mn(PO₄)₂, Mg₃Fe₃(PO₄)₄,MgCO₂(PO₄)₂, Mg₃Co₃(PO₄)₄, MgNi₂(PO₄)₂ MgMnFe(PO₄)₂ MgMnCo₂(PO₄)₂. Anolivine structured material can be synthesized through a variety ofmethods, such as by + chemically or electrochemically removing Li⁺ fromLiFePO₄ and then reacting the resulting material FePO₄ with Mg. Thesemethods enable proper site ordering of the Mg onto the Li sites. Inother embodiments, the material is prepared by direct solid statesynthesis from Mg-containing precursors.

In certain embodiments, the magnesium materials can have eitherrhombohedral or monoclinic NASICON (Na₃Zr₂Si₂PO₁₂) structures, where the“Zr” site is at least partially occupied by a transition metal Ti, V,Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Sn, Sb, the “Si” and “P” sites areoccupied by Al, P, Si, V, or As, and the “O” site is occupied by O.Magnesium may be located at a number of low-energy “Na” sites throughoutthe material. Exemplary materials include Mg_(0.5)V₂(PO₄)₃, MgV₂(PO₄)₃,Mg_(0.5)Ti₂(PO₄)₃, Mg_(1.5)Cr₂(PO₄)₃. In some embodiments, the compoundsas described herein has a unit cell atomic arrangement isostructuralwith a monoclinic or rhombohedral NASICON unit cell.

In certain embodiments, the material is isostructural with the LiVP₂O₇structure, where the “Li” site is fully or partially occupied bymagnesium, the “V” site is fully or partially occupied by a transitionmetal, the “P” site is occupied by “P”, and the “O” site is occupied by“O”. In particular embodiments, the LiVP₂O₇-based structure includes oneor more of Ti, V, Cr, Mn, Fe, Mo, or mixtures thereof on the “V” site.

In certain embodiments, the material is isostructural with vanadiumoxy-phosphate VO(PO₄) structure. VO(PO₄) is a known oxy-phosphate thatoccurs in several polymorphic structures (alpha, beta, delta, omega,epsilon, gamma), as both the hydrated and dehydrated forms. In someembodiments, the material is isostructural with the hydrated anddehydrated forms of alpha, beta, delta, omega, epsilon, gamma VOPO₄compounds. In some specific embodiments, the compound has a structuredisplaying electrochemical properties and Mg mobility and are used as anMg insertion electrode. Among the vanadium oxy-phosphate compounds, thebeta form of VOPO₄ displays excellent stability upon magnesium insertionand removal and excellent capacity, up to one magnesium ion per vanadiumatom. In some embodiments, the compound has a unit cell atomicarrangement isostructural with a beta-VOPO₄ unit cell. In someembodiments, the compound is beta-VOPO₄.

In yet another embodiment, magnesium can be chemically orelectrochemically inserted into metal diphosphate compounds with thecubic TiP₂O₇ structure. Non-limiting examples include TiP₂O₇ and MoP₂O₇.Compounds with the VP₂O₇ structure, such as VP₂O₇, can also insertmagnesium though at a moderately slower rate than the cubicdiphosphates. Other compounds contemplated within the diphosphate classinclude Mg_(0.5)TiP₂O₇, Mg_(0.5)VP₂O₇, Mg_(0.5)CrP₂O₇, Mg_(0.5)MoP₂O₇,Mg₁CrP₂O₇, Mg₁MnP₂O₇ Mg₁CoP₂O₇, Mg₁NiP₂O₇. In some embodiments, thecompound has a unit cell atomic arrangement isostructural with a cubicdiphosphate TiP₂O₇ unit cell.

In some specific embodiments, the compound is olivine MgFe₂(PO₄)₂. Inother specific embodiments, X is P. In still other specific embodiments,the compound is NASICON MgFe₂(PO₄)₃ or MgV₂(PO₄)₃.

In some embodiments, b′ is not 0 and the dopant A partially substitutefor the transition metals to enhance the performance or cost of theelectrode material. Batteries containing magnesium anodes and cathodescomprising of these materials have also higher theoretical energydensity and specific energy than current commercial lithium-ionbatteries, specified by a carbonaceous insertion anode.

Synthetic Methods

In yet another aspect, the method of synthesizing a compound asdescribed herein is disclosed, using solid state synthesis,co-precipitation, or carbothermal reduction from magnesium containingprecursor, wherein the magnesium containing precursor is one or morecompounds selected from the group consisting of MgO, Mg(OH)₂, MgCO₃,MgSO₄, MgS, MgF₂, MgHPO₄, Mg metal, and mixtures thereof.

In some embodiments, the materials described herein as Mg electrodematerials by general formulas layered Mg_(a)M_(b)X_(y), spinelMg_(a)M_(b)X_(y) and polyanion Mg_(a)M_(b)(XO_(z))_(y) are prepareddirectly from Mg-containing precursors (oxide, hydroxide, carbonate,oxalate, sulfate, etc.) by means of either solid state reaction,co-precipitation from solution, or carbothermal reduction of solidprecursors. For example, MgMn₂O₄ spinel can be directly synthesizedusing ceramic methods such as solid-state reaction of 1MgO (0.403 g) and2Mn₂O₃ (1.58 g). The powders are first mixed with mortar and pestle, orpreferably with shaker or planetary milling to enable thoroughdispersion of the precursors. Subsequently, the sample is heated as apowder, or preferably first pelletized to facilitate solid statediffusion, then heated in air, nitrogen, or argon gas, at temperaturesof 500 to 700° C. The above reaction conditions also enable the use ofother Mg-precursors such as Mg(OH)₂+Mn₂O₃ to form MgMn₂O₄ and H₂O vapor.In other embodiments, the electrode material MgMn₂O₄ is directlysynthesized using co-precipitation from aqueous solution in order toobtain small particle size. In some embodiments, MgSO₄ and MnSO₄*H₂O aredissolved in a 1 to 2 molar ratio in water to create about 1 M totalsolution of the precursors while bubbling nitrogen gas through thesolution. To that, ˜4.5 mols NaOH in concentrated aqueous solution (˜2.5M solution) add dropwise, all while bubbling air through the solutions,so as to produce hydroxides. Subsequently the precipitates are collectedon filter paper and washed with de-ionized water, followed by drying at50 to 100° C. (preferably 70° C.) in air. Thereafter it is possible togrow the crystal grains by annealing the sample between 300° C. and 700°C.

In some embodiments, during the synthesis of Mg-containing spinelmaterials as described above MgO can sometimes be obtained as animpurity phase (whether synthesizing oxides or sulfides etc.), which canbe quite electronically and ionically insulating. A further method ofmaking the magnesium spinel materials seeks to avoid or reduce the MgOformation. A method of preparing spinel materials for Mg insertion firstrequires synthesis of the Cu, Zn, Ag, Li, Na, K, or Ca form of thematerial, and then extracts the Cu, Zn, Ag, Li, Na, K, or Ca from thematerial (both by electrochemical charging in an electrode of a Mg or Licell, or by chemical extraction by acid or oxidizer) and replacing itwith Mg by chemical or electrochemical means. In some embodiments,ZnMn₂O₄ is synthesized using co-precipitation from aqueous solution ofZnSO₄ and MnSO₄*H₂O, and then Zn is extracted from by means of nitricacid so as to avoid MgO formation. In other embodiments, solid statereaction is used. In some specific embodiments, CuS and Cr₂S₃ arereacted under inert atmosphere, or sealed ampoule up to temperaturebetween 600° to 1000° C. to form CuCr₂S₄ spinel. Subsequently Cu ischemically or electrochemically removed, and Mg is chemically orelectrochemically reinserted.

In some embodiments, a method of synthesizing a compound describedherein is disclosed, wherein a precursor containing one or more metalsselected from the group consisting of Cu, Zn, Ag, Na, K, Rb, Cd, Ca, Sr,Ba, and a combination thereof is used; and the Cu, Zn, Ag, Na, K, Rb,Cd, Ca, Sr, or Ba in the precursor is chemically or electrochemicallyextracted and replaced with Mg by chemical or electrochemical insertion.In some specific embodiments, the preparation of the above layered orspinel materials is accomplished using the two-step sequence in which anintermediate compound is synthesized with Na, K, Li, Cu, Ag, Zn, Caoccupying the place of Mg in the tetrahedral spinel site, or theoctahedral site of the Mg layer in layered materials. Subsequently, theplace-holding cation (Na, K, Li, Cu, Ag, Zn, Ca) from the tetrahedralsite of the spinel, or the octahedral site of the Mg layer in layeredmaterials, can be chemically extracted by immersing the intermediatephase in a chemical oxidant or acidic solution (e.g. pH=˜1 HCl) forbetween 6-24 h at room temperature with stirring. Subsequently, thesample is washed with deionized water, dried at room temperature andreduced pressure (10⁻³ Torr), or dried at 60° C. to 120° C. underambient or reduced pressure (10⁻³ Torr). Mg ions are then inserted intothe vacant tetrahedral site first by preparing an electrode containingthe spinel material, a conductive carbon (e.g. Super P), and binder(e.g. PVdF) which is placed in an electrochemical cell with Mg metalanode, Mg alloy anode, or lower voltage Mg-containing insertionmaterial, and an Mg-conducting electrolyte. Subsequent dischargecorresponds to Mg insertion into the now vacant tetrahedral site of thespinel structure, or the vacant Mg octahedral site of layered materials,thus enabling high Mg diffusion into the host structure. AlternativelyMg ions are inserted into the now vacant tetrahedral site of the spinelstructure, or the vacant Mg octahedral site of layered materials, bychemical insertion resulting from immersion of the sample in a solutionof 300% excess di-butyl magnesium in heptane and stirring attemperatures between 20° C. to 40° C. for 5 days to 2 weeks.

In other embodiments, an electroactive Mg layered or spinel material issynthesized. In these specific embodiments, the intermediate compoundhaving Na, K, Li, Cu, Ag, Zn, Ca or mixtures thereof occupying the placeof Mg in the tetrahedral spinel site or the octahedral Mg site of thelayered materials are used as electrode active material in an electrodein cell. The electrode also comprises conductive carbon black, and PVdFbinder. The cell comprises additionally a non-aqueous Li-ion conductingelectrolyte, and Li metal anode. The cell is galvanostatically chargedto a voltage that ensures extraction of the place-holding cation (e.g.4.3 V vs. Li LiMn₂O₄) and potentiostatically held at the same voltagefor about 10 h, or until the charge capacity is between about 80% and100% of the theoretical value (e.g. 125 mAH/g and 150 mAh/g forLiMn₂O₄). At this point the cell is dismantled, and the remaining Mn₂O₄cathode is washed with dimethyl carbonate or di-ethyl carbonate toremove residual Li salt. Subsequently the electrode is dried at eitherroom temperature and reduced pressure (10⁻³ Torr) or 60° to 120° C.under ambient or reduced pressure (10⁻³ Torr). Mg ions can be insertedinto the now vacant tetrahedral site of the spinel structure, or thevacant Mg octahedral site of layered materials, thus enabling high Mgdiffusion into the host structure. by placing that electrode in anelectrochemical cell with Mg metal anode, Mg alloy anode, or lowervoltage Mg-containing insertion material, and an Mg-conductingelectrolyte. Subsequent discharge corresponds to Mg insertion into thenow vacant tetrahedral site of the spinel structure, or the vacant Mgoctahedral site of layered materials, thus enabling high Mg diffusioninto the host structure. Alternatively Mg ions can be inserted into thenow vacant tetrahedral site of the spinel structure, or the vacant Mgoctahedral site of layered materials, by chemically inserting Mg ions byimmersing the sample in a solution of 300% excess di-butyl magnesium inheptane and stirring at temperatures between 20° to 40° C. for 5 days to2 weeks.

In some embodiments, a variety of Mg-containing polyanion electrodematerials are prepared from direct synthesis of Mg-containingprecursors. In other embodiments, the Mg-containing polyanion electrodematerials are synthesized using a the two-step sequence in which anintermediate compound is synthesized with Cu, Zn, Ag, Li, Na, K, or Caoccupying the Mg site. Subsequently the place-holding cation (i.e. Cu,Zn, Ag, Li, Na, K, or Ca) is chemically or electrochemically removed,and Mg is chemically or electrochemically inserted into the now vacantsite. In some embodiments, olivines, or other polyanions, formed fromdivalent Mg and a divalent transition metal will form with high degreeof mixing between the intended Mg-site and the divalent transition metalsite due to the isovalent nature of the cations. Such mixing cansignificantly inhibit Mg diffusion within the host structure. Forexample the work of Nuli et al. on Mg insertion intoMg_(1.03)Mn_(0.97)SiO₄ demonstrates that appreciable site disorder(8-11% in their work) limits them to obtaining less than ½ of thetheoretical capacity for this material. The use of monovalent Li, Na, K,Cu, Ag, to induce separation of the cations into separate lattice sites,can reduce such issues. In general, the isovalent mixing may occur to agreater degree when using Mg-containing precursors to prepare olivinesof Mg²⁺, M²⁺, and (SiO₄)²⁻ than when preparing other oxy-anionsmaterials (e.g. phosphates, arsenates, sulfates).

Similarly, the NASICON structure of Mg_(0.5)Ti₂(PO₄)₃ studied by Makinoet al. J Power Sources 112, 85-89, 2002) demonstrates subtle change inthe lattice parameter (and therefore unit cell volume) when preparingthe material with substitution of Ti with Fe or Cr, which correspondswith limited Mg migration due to Mg trapping in some lattice sites. Insome embodiments, certain NASICON materials display more preferablelattice parameters when prepared via the two-step reaction sequencedescribed herein, which requires the intermediate synthesis of Li, Na,K, Cu, Zn, Ag, compounds with these place-holding cations occupying theMg site. Subsequently the place-holding cation is chemically orelectrochemically removed and Mg is chemically or electrochemicallyinserted as described in the previous paragraphs. In some specificembodiments, Li-titanium NASICON displays slightly larger cell volumethan the Mg-titanium NASICON (226.27 Å³ for Li₁Ti₂(PO₄)₃ vs. 225.86 forMg_(0.5)Ti₂(PO₄)₃). Accordingly, the Li-compound is prepared, then Li isreplaced with Mg, resulting in compounds with more favorable Mg mobilitythan the directly synthesized Mg-compound. In an opposing example, theMg-vanadium NASICON, which has not been previously reported for use asan Mg insertion electrode, displays slightly larger unit cell volumewhen preparing it directly from Mg-containing precursors thanLi-containing precursors (217.50 Å³ for Li₁V₂(PO₄)₃ vs. 221.79 forMg_(0.5)V₂(PO₄)₃), which indicates to us that direct synthesis fromMg-containing precursors rather than Li (or one of the other secondarypreferable cations) should not lead to significant differences in the Mgmobility.

Methods to Prepare a Magnesium Ion Battery

In yet another aspect, an electrode for a magnesium battery isdescribed, comprising a compound of formula A_(b′)Mg_(a)M_(b)X_(y) orA_(b′)Mg_(a)M_(b)(XO_(z))_(y) as described herein. In some embodiments,the compound of formula A_(b′)Mg_(a)M_(b)X_(y) is not layered VS₂,spinel Co₃O₄, layered V₂O₅, rocksalt MnO, spinel Mn₂O₄, spinel Mn₃O₄, orlayered ZrS₂. In some embodiments, the compound of formulaA_(b′)Mg_(a)M_(b)(XO_(z))_(y) is not Mg_(a)MnSiO₄ or Mg_(0.5)Ti₂(PO₄)₃.In some embodiments, these compounds are used as cathode activematerial. In some embodiments, these compounds are used as anode activematerial. In some embodiments, the compound has a layered structure. Inother embodiments, the compound has a spinel structure.

In some embodiments, the electrode further comprises an electronicallyconductive additive. Non-limiting examples of electronically conductiveadditive include carbon black. In some embodiments, the electrodefurther comprises a binder. Non-limiting examples of binder includepolyvinylidene fluoride, polytetrafluoroethylene, and terpolymer orcopolymer thereof.

In some embodiments, an Mg-metal or Mg alloy is used as an anode whichresults in significant gains in volumetric and gravimetric energydensity compared to lithium-insertion anodes. While gravimetric energycontent is important for electrical vehicle applications, volumetricconsiderations have been an even larger concern for large battery packsand small battery packs for portable electronics. The high capacity ofmagnesium and the use of Mg-metal, or Mg alloy, anodes can provideenergy densities approaching 1600 Wh/l, which would make magnesiumbatteries one of the highest energy density technologies available.Furthermore, the ready availability of magnesium from a variety of rawmaterials sources can significantly lower cost of production.

A non-aqueous electrolyte solution is prepared by dissolving a magnesiumsalt in an appropriate solvent. Exemplary magnesium salts include MgCl₂,Mg(ClO₄)₂, Mg(SO₂CF₃)₂, Mg(BF₄)₂, Mg(CF₃SO₃)₂, and Mg(PF₆)₂. Exemplarynon-aqueous solvents include propylene carbonate, ethylene carbonate,butylene carbonate, vinylene carbonate, gamma-butyl lactone, sulfolane,1,2-dimethoxyethane, 1,2-diethoxyethane, 2-methyltetrahydrofuran,3-methyl-1,3-dioxolane, methyl propionate, methyl butyrate, dimethylcarbonate, diethyl carbonate and dipropyl carbonate. The foregoingnonaqueous solvent may be employed solely or a plurality of materialsmay be mixed. It is preferable that cyclic carbonate or chain carbonateis employed from a viewpoint of realizing electric stability. In othercases the non-aqueous electrolyte solution may be composed wholly or inpart of an organo-Magnesium complex solution. Such solutions consist ofa Grignard compound, composed of Magnesium coordinated to both anorganic ligand and a halide dissolved in one of the exemplary solventsdescribed above. Some exemplary examples of the organic ligands includealkyl, aryl, alkenyl, heteroaryl, or n-dibutyl groups, and the halidesinclude F, Cl, Br, I. In some cases such a compound is also complexedwith a strong Lewis Acid such as AlBr₃, AlCl₃, BCl₃, BF₃, FeCl₃, FeBr₃,TiCl₄, SnCl₄, in one of the exemplary solvents described above.

In yet another aspect, an energy-storing device is described,comprising:

a first positive electrode comprising the compound of formulaMg_(a)M_(b)X_(y) or Mg_(a)M_(b)(XO_(z))_(y) as described herein; and

a second electrode comprising a magnesium metal, a magnesium alloy, orthe compound of formula Mg_(a)M_(b)X_(y) or Mg_(a)M_(b)(XO_(z))_(y) asdescribed herein.

In some embodiments, the energy-storing device is a Mg battery. In somespecific embodiments, the Mg battery can be prepared as follows. Asuitable Mg cathode material selected from the materials describe hereinis combined with a conductive materials such as carbon black, graphiteor other conductive carbon and a binder to generate a uniform mixture.The positive electrode is pressed and/or molded into a desired shaper oris extruded onto a current collector.

The negative electrode, typically Mg metal, Mg alloy, or a secondelectrode selected from the materials describe herein, and the positiveelectrode are positioned on opposite sides of a separator typicallyconstituted by a porous film made of polypropylene in a battery can. Thenon-aqueous electrolytic solution is introduced into the battery can andis sealed, for example, by crimping.

In some embodiments, the negative electrode and the positive electrodeare again accommodated in the battery cell, which can take on a varietyof standard forms including, but not limited to cans, stacks, laminates,rolls, etc. Then the separator is disposed between the negativeelectrode and the positive electrode and, new non-aqueous electrolyticsolution is introduced into the battery cell and the can is crimped toclose.

Example 1 Tables 1 and 2 of Ab Initio Results for Target Materials

Tables 1 and 2 summarize the composition, structure, and calculatedproperties for a number of magnesium containing compounds. The compoundsexhibit the structure and composition necessary to provide reasonablereaction voltage and Magnesium mobility as computed using a densityfunctional theory framework using the Generalized Gradient Approximation(GGA) with or without a Hubbard correction term (GGA+U), depending onthe chemistry involved. The mobility barriers provided in Tables 1 and 2refer to the minimum activation barrier for an Mg²⁺ ion to traverse theentire compound unit cell. An activation barrier is defined as thedifference in energy between two distinct Mg crystallographic sites andis computed by elastic band calculations. A detailed description of themethod and its accuracy can be found in [G. Mills, H. Jonsson, and G. K.Schenter, Surf. Sci. 324, 305, 1995.]. Materials with barriers <800 meVcorrespond to materials predicted to exhibit useful rates of topotaticMg insertion and removal. Furthermore, consider a Mg insertion reactionwith a cathode compound of the formula M_(b)X_(y) in the de-maggedstate:

M_(b)X_(y) +aMg

Mg_(a)M_(b)X_(y)  [3]

The average voltage in a Mg insertion reaction is computed by groundstate relaxation calculations of the starting compound state(M_(b)X_(y)), the inserted compound state (Mg_(a)M_(b)X_(y)) and the Mgmetal state. The voltage (as given in the table in Tables 1 and 2) canthen be computed according to the following formula:

V═(E(Mg_(a)M_(b)X_(y))−E(M_(b)X_(y)))/(aE(Mg))  [4]

where E denotes the ground state total energy as computed by densityfunctional theory methods [M. K. Aydinol, A. F. Kohan, and G. Ceder inAb Initio Calculation of the Intercalation Voltage of Lithium TransitionMetal Oxide Electrodes for Rechargeable Batteries, Elsevier Science Sa,New York, 1997, pp. 664-668.]. The voltages correspond to a range ofabout 0.5 V vs. Mg to 3.75 V vs. Mg thus illustrating that the varietyof materials comes from multiple categories of materials; classifiedhere as polyanion, layered, and spinel compounds. The accuracy of thepredictive methods has been validated in numerous publications, such asZhou et al [F. Zhou, M. Cococcioni, C. A. Marianetti, D. Morgan, G.Ceder, Physical Review, B 70 (2004) 235121] and Jain et al [Jain et al,Computational Materials Science, 50, 2295-2310, 2011].

Table 2 illustrates a comparison of the mobility barriers for Mg²⁺ andLi⁺ for several compounds. As Table 2 shows, there exists a non-trivialrelationship between the size and valence of the moving ion and thestructure and chemistry of the host compound, which determines the ionicmobility of the specific ion in that compound. Specifically, the resultin Table 2 shows that one cannot expect an ionic cathode material withgood Li⁺ diffusivity to automatically exhibit good Mg²⁺ diffusivity.Conversely, there may be compounds with sluggish Li⁺ diffusivity whichexhibit good Mg²⁺ diffusivity.

TABLE 1 Mg insertion compounds with assigned properties calculated by abinitio methods Average Ab Reaction De-Magged initio Voltage CountComposition Category barrier vs. Mg 1 FeCl₃ Layered 619 2.34 (1e−) 2V₂(PO₄)₃ Polyanion 536 2.63 (3e−) NASICON Trigonal 3 Cr₂S₄ Spinel 5531.67 (2e−) 4 Mn₂O₄ Spinel 575 2.90 (2e−) 5 CrS₂ Layered 788 1.51 (2e−) 6FePO₄ Olivine Polyanion 608 2.66 (1e−) 7 Ni₂O₄ Spinel 626 3.52 (2e−) 8CrMo(PO₄)₃ Polyanion 591 2.42 (3e−) NASICON Trigonal 9 CoO₂ Layered 6743.18 (0.5e−) 10 Mo₂(PO₄)₃ Polyanion 844 2.21 (3e−) NASICON Monoclinc 11MoPO₄ Polyanion 853 2.16 (2e−) 12 (CoMn)O₄ Spinel 938 2.54 (2e−) 13TiP₂O₇ Polyanion 449 0.61 (1e−) 14 VS₂ Spinel Spinel 497 0.94 (2e−) 15V₂O₄ Spinel Spinel 504 2.25 (2e−) 16 TiO₂ Anatase Anatase 547 0.67 (1e−)17 Alpha-VOPO₄ Polyanion 548 2.37 (2e−) 18 V₁₈O₄₄ Pseudo-Layered 1562.37 (12e−) 19 Beta-VOPO₄ Pseudo-Layered 774 2.58 (2e−) 20 FeOCl Layered389 1.54 (1e−) 21 V₂O₅ (de-Magged Pseudo- 545 2.58 (2e−) MgV₂O₅) Layered22 V₂O₅ (Common Pseudo-Layered 564 2.43 (2e−) Ground State)

TABLE 2 Mg insertion compounds with assigned properties calculated by abinitio methods Ab Average initio Reaction Ab initio barrier Voltage vs.Count De-Magged Composition Category barrier (Mg) (Li) Mg 1 FeCl₃Layered 619 172 2.34 (1e−) 2 V₂(PO₄)₃ NASICON Trigonal Polyanion 536 6142.63 (3e−) 3 Cr₂S₄ Spinel 553 380 1.67 (2e−) 4 Mn₂O₄ Spinel 575 227 2.90(2e−) 5 CrS₂ Layered 788 392 1.51 (2e−) 6 FePO₄ Olivine Polyanion 608237 2.66 (1e−) 7 Ni₂O₄ Spinel 626 415 3.52 (2e−) 8 CrMo(PO₄)₃ NASICONTrigonal Polyanion 591 409 2.42 (3e−) 9 CoO₂ Layered 674 580 3.18(0.5e−) 10 Mo₂(PO₄)₃ NASICON Monoclinc Polyanion 844 293 2.21 (3e−) 11MoPO₄ Polyanion 853 74 2.16 (2e−) 12 (CoMn)O₄ Spinel 938 585 2.54 (2e−)13 TiP₂O₇ Polyanion 449 297 0.61 (1e−) 14 VS₂ Spinel Spinel 497 228 0.94(2e−) 15 V₂O₄ Spinel Spinel 504 504 2.25 (2e−) 16 TiO₂ Anatase Anatase547 511 0.67 (1e−) 17 Alpha-VOPO₄ Polyanion 548 220 2.37 (2e−) 18 V₁₈O₄₄Pseudo- 156 95 2.37 (12e−) Layered 19 Beta-VOPO₄ Pseudo- 774 292 2.58(2e−) Layered 20 FeOCl Layered 389 677 1.54 (1e−) 21 V₂O₅ (de-MaggedMgV₂O₅) Pseudo- 545 194 2.58 (2e−) Layered 22 V₂O₅ (Common Ground State)Pseudo- 564 260 2.43 (2e−) Layered

Example 2 Description of Spinel Synthesis Example 2a Co-PrecipitationMethod of MgMn₂O₄ and MgNiMnO₄

This example demonstrates the synthesis of a high mobility compound witha low diffusive path for magnesium, a new synthetic method was developedwhich involved a co-precipitation of hydroxide salts, followed by a lowtemperature calcination process according to the following reactions.

MgSO_(4(aq))+2MnSO_(4(aq))+(excess)NaOH_((aq))→Mg(OH)₂(s)+Mn(OH)_(2(s))+Na₂SO_(4(aq))(25°C.)  [5]

Mg(OH)_(2(s))+2Mn(OH)_(2(s))→nano-MgMn₂O₄(300° C./8 hr)  [6]

In a typical synthesis, 2.40 g of MgSO₄ and 6.75 g of MnSO₄.H₂O weredissolved in 100 ml of HPLC water at room temperature in a≦500 mlErlenmeyer flask. Subsequently, 200 ml of a 1M NaOH solution was slowlyadded dropwise over 15 minutes with continual stirring at roomtemperature. Immediately, a brown precipitant formed with the causticaddition and thus was critical to continually stir the solution duringthe addition to minimize agglomeration. Stirring of the solution andproduct at room temperature continued for 24 hours on a stir plate toensure reaction completion. The brown precipitant was isolated bycentrifugation (2000 rpm/15 min) and worked-up through several waterrinses until the rinsate was neutral to pH paper. Then, the nano-MgMn₂O₄powder was vacuum dried at 100° C. for 2 hours. Finally, the vacuumdried powder was annealed at several temperatures in a muffle furnace,namely 300° C., 500° C., and 750° C. in order to show thecrystallization progression and purity of the sample. FIG. 3 depicts aresultant X-Ray diffraction spectrum from the synthetic route appliedand confirms the method affords pure phase, nanocrystallite (20-25 nm)MgMn₂O₄ material. Similar synthetic methods involving co-precipitationof hydroxide salts to yield high mobility Mg compounds were created forother compounds described herein. For example, MgNiMnO₄ spinel is madein a similar fashion according to the following reactions:

MgSO_(4(aq))+2MnSO_(4(aq))+NiSO_(4(aq))+xsNaOH_((aq))Mg(OH)_(2(s))+Mn(OH)_(2(s))+Ni(OH)_(2(s))+Na₂SO_(4(aq))(25°C.)  [7]

Mg(OH)_(2(s))+Mn(OH)_(2(s))+Ni(OH)_(2(s))→“MgNiMnO₄”-nano(300° C./6hr)  [8]

Thereafter, confirmation of the intended compound, MgNiMnO₄ spinel, isexhibited in FIG. 4.

Example 2b Solid State Synthesis of ZnCr₂S₄

Spinels of other chemical classes have also been prepared for materialsto be used in Mg electrode active materials. Zinc chromium sulphide wassynthesized by solid state reaction methods using stoichiometric amountsof ZnS powder (0.655 g) and Cr₂S₃ (1.345 g) powder according to thefollowing reaction:

ZnS+Cr₂S₃→ZnCr₂S₄900° C./12 hr/Argon/tube furnace  [9]

The mix was pelletized and placed in a tube furnace with flowing argonfor 12 hours. XRD analysis showed the product to be relatively phasepure ZnCr₂S₄ (>95%) as demonstrated in FIG. 5. For those with expertisein the art, synthesis of a variety of high mobility spinel materials canbe envisioned using such techniques.

Example 3 Chemical De-Mag of MgMn₂O₄ (Magnesium Manganese Oxide Spinel),Electrochemical De-Mag of MgMn₂O₄

This example typifies the ability to prepare Mg containing compounds,and remove the Mg through chemical or electrochemical means in atopotactic fashion. FIG. 1 is an X-ray diffraction (XRD) spectra ofMgMn₂O₄ spinel demonstrating (1) chemical Mg extraction and (2)subsequent electrochemical magnesium insertion according to one or moreembodiments. This is the X-Ray diffraction spectra corresponding to asample of MgMn₂O₄ spinel, which was synthesized, then immersed in acidto remove Mg. Following that the sample was placed in an electrochemicaltest cell to electrochemically reinsert Mg. FIG. 2 is a voltage profilecorresponding to MgMn₂O₄ spinel in a electrochemical test celldemonstrating the higher activity observed by electrochemicallyextracting Mg from the tetrahedral sites of MgMn₂O₄ spinel at highvoltage, than when attempting to insert additional Mg into the spinel toform a rock salt related compound at low voltage.

Example 4 Synthesis of Polyanions: Lithium Iron Phosphate (LFP) toChemically De-Lithiated LFP

Lithium Iron phosphate was prepared according to two step reaction asdetailed in equations 5,6:

Li₂CO₃+Fe(C₂O₄).2H₂O+NH₄H₂PO₄→‘mechanochemical-reacted precursormix’  eq5

Conditions: 1) ball milling in Acetone 12 hr@RT followed by 2)decanting/removing acetone

‘Precursor mix’→LiFePO₄+H₂O+N₂/H_(2(g))+COx(g)  [10]

Conditions: 1) 350 C/10 hr/Argon annealing followed by 2)600 C/10hr/Argon calcination

The first step involves a mechano-chemical process to yield a precursormix, followed by a second step, in which the precursor mix material isannealed at two different temperatures. In a standard example, 0.937 gof Li₂CO₃, 4.10 g of FeC₂O₄.2H₂O, and 2.77 g of NH₄H₂PO₄ were premixedand placed in a 500 ml plastic container. Approximately 200 ml ofacetone is added along with 2, 3, & 10 mm diameter ZrO₂ ball media tothe plastic container. Subsequently, the container with the material wasplaced on a rolling mill and milled at room temperature for 12 hours.Work-up of the mechanochemical precursor mix involved both decantationand removal of acetone with heat. Next, the product was pelletized andplaced in a tube furnace at 350° C. for 10 hours under flowing argon inorder to removed residual volatile components. Finally, after recoveringthe product from the 350° C. anneal step, the material was re-pelletizedand placed back into the tube furnace at 600° C. for 10 hours underflowing argon. The resultant product was structurally characterizedusing powder XRD and was confirmed to be triphylite, LiFePO₄ (FIG. 6).In some embodiments, FePO₄ is used as a magnesium insertion cathodematerial.

Chemical de-lithiation of LiFePO₄ involved the use of a strong oxidant,namely potassium persulfate as shown in equation 7.

LiFePO₄+(X/2)K₂S₂O₈→(1−X)LiFePO₄+(X)FePO₄+X/2(Li2SO₄+K2SO4)  [11]

(0>X>1)

A typical de-lithiation example involved adding 2.00 g of K₂S₂O₈ with100 ml of HPLC water in a 250 ml Erlenmeyer flask, until fullydissolved. Subsequently, 0.60 grams of LiFePO4 was added to the solutionand stirred for 24 hours at room temperature. Note, that the amount ofpersulfate was purposely added in excess to ensure reaction completion.After the 24 hours, the product was rinsed with copious amounts of HPLCwater and vacuum dried. FIG. 7 illustrates an XRD spectral comparison ofthe starting LFP material versus the material after chemicalde-lithiation.

Example 5 Chemical Magging of VOPO₄ and FePO₄

This is an example illustrating the chemical Magging of polyanioncompounds. FIG. 8 contains the XRD spectra demonstrating the synthesisof the polyanion compound vanadium oxy-phosphate, VOPO₄, followed bychemical insertion of Mg by immersing the dried powder in a solution ofButyl Magnesium in heptanes. The solution was stirred at about 30° C.for 6 days prior to removing the sample for X-Ray diffraction. The lowangle peaks shifting to lower 2 theta diffraction angles correspond withtopotactic Mg insertion into VOPO₄.

Example 6 Electorchemical Magging of FePO₄ in Polyanion Compounds

FIG. 9 illustrates the electrochemical Mg insertion into FePO₄ inmultiple Mg containing electrolytes. In each case the starting materialwas LiFePO₄ as the active material prepared as a composite electrodewith conductive carbon additive, and fluoropolymer as binder. Prior toimmersing the electrodes in Mg-ion electrolyte, the LiFePO₄ electrodewas charged and discharged several times an electrochemical cell inorder to remove the Li (i.e., leaving the electrode in the chargedstate). Thereafter the FePO₄ electrode is washed with solvent of likekind and moved to an electrochemical cell containing an Mg-ionconducting electrolyte. FIG. 9 illustrates the voltage measured vs. timeduring the charge and discharge of FePO₄ (post removal of Li) in 3 typesof Mg-ion conducting electrolytes. The first consists of “APC” theorgano-Mg electrolyte of 0.25M Phenyl Magnesium Chloride: AluminumChloride (in 2:1 mol ratio), which enables the use of an Mg metal anode(thus the voltage is presented as V vs. Mg). The second and third Mg-ionconducting electrolytes are similar to one another. The are 0.5 MMg(ClO₄)₂ in Propylene Carbonate, “PC”, and 0.5 M Mg(ClO₄)₂ inAcetonitrile, “ACN.” Being similar electrolytes, which do not enable thestraightforward use of an Mg metal anode, the reaction voltages areplotted here with reference to silver (Ag) metal (utilized as apseudo-reference electrode to monitor the extent of the electrochemicalreaction). For the purposes of these tests 0 V vs. Ag is equal to about2.25 V vs. Mg+/−0.25 V. Each of these 3 Mg-ion conducting electrolytesdemonstrates the possibility of charging and discharging cellscontaining active materials described herein to electrochemically insertand remove Mg from these electrode active materials in a reversiblemanner.

Example 7 Synthesis of Layered and Pseudo-Layered Compounds: CuCrS₂,FeOCl and V₁₈O₄₄ CuCrS₂

Copper chromium sulfide, CuCrS₂ is considered a layered compound withanionic chromium sulfide sheets separated by copper cations. CuCrS₂ wassynthesized via a salt flux method according to the reaction outlined inequation 12:

Cu₂S+Cr₂S₃→CuCrS₂NaCl flux/800° C./12 hour  [12]

A typical reaction involved hand mixing 1.35 g of Cu₂S with 2.84 g ofCr₂S₃ and 10.0 g of NaCl and placing the mixture in an Inconel boat.(Note, the NaCl was predried in a vacuum oven for 1.5 hours before use)The mixture was then placed in a tube furnace and heated to 800° C. for12 hours under flowing argon. The product was recovered from the Inconelboat through multiple rinses with water to remove the salt and driedunder dynamic vacuum at 80° C. for 2 hours. FIG. 10 shows an XRDcharacterization of the material produced, which indexed with anexcellent figure of merit to the layered CuCrS₂ compound.

FeOCl

Iron oxychloride, FeOCl is a layered compound and was prepared followingmethod as described in equation [13]:

α-Fe₂O₃+FeCl₃→3FeOCl Sealed glass tube/370° C./2 days  [13]

A typical reaction involved hand mixing 0.156 g of alpha-Fe₂O₃ with0.344 FeCl₃ and charging a glass ampoule (20 cm long×2.0 cm dia×2.0 mmwall thickness). The glass ampoule was vacuum sealed using standardtechniques known in the art and placed in a muffle furnace, heated at370° C. for 48 hours. The formation of dark red-violet plate-likecrystals indicated FeOCl formation. Product was first washed with HPLCwater to remove any residual FeCl₃, then washed with acetone and dried,noting that prolonged exposure to moisture causes FeOCl to hydrolyze.FIG. 11 shows an XRD characterization of the material produced, whichindexed with an excellent figure of merit to the layered FeOCl compound.

V₁₂O₂₉

Vanadium oxides with the stoichiometry V₁₈O₄₄ and V₁₂O₂₉ can beconsidered similar layered structures with unique properties as comparedto the V₂O₅ parent layered structure. Experimentally the approachtowards targeting these unique materials was to synthesize the lithiumsalts, namely LixV₁₈O₄₄ and Li_(x)V₁₂O₂₉ (x=0-3.5). A typical reactionof Li_(x)V₁₂O₂₉ (or Li_(x)V₁₈O_(43.5)) involved first premixing 0.382 gof LiV₃O₈ (made via a solid state process as reported in literature)with 0.392 g of V₂O₅ and 0.583 g of ammonium metavanadate, NH₄VO₃. Themixture was then placed in an alumina crucible and fired at 350° C. for12 hours followed by an additional calcination for 12 hours at 650° C.After recovering the product, 0.5 g of the material was placed in abeaker with 50 ml of 0.5M HCl and stirred for 4 hours at roomtemperature. The product was then rinsed with HPLC water to neutral pHand finally calcined for 4 hours at 650° C. The aforementioned processyields Li_(x)V₁₂O₂₉ (x=0-3.5) crystalline powder in fairly high purityas seen in FIG. 12. Structural analysis involved comparing the productto known sodium analogs, namely Na_(0.6)V₁₂O₂₉, and to theoretical XRDpatterns of topotatic, lithium-inserted V₁₂O₂₉ structures. Elementalanalysis for exact lithium content was not performed.

1. A magnesium battery electrode comprising: a current collector, and anelectroactive compound in electronic communication with the currentcollector, the electroactive compound having the formula ofA_(b′)Mg_(a)M_(b)(XO_(z))_(y), wherein A is one or more dopants selectedfrom the group consisting of Al, Li, Na, K, Zn, Ag, Cu, and mixturesthereof; M is one or more transition metals selected from the groupconsisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Sn, Sb, Bi, Ta,W, and mixtures thereof; X is one or more anions selected from the groupconsisting of P, V, Si, B, C, As, S, N, and mixtures thereof; 0≦b′≦3.9;0<a≦3.1; 0.9≦b≦3.9; 0.9≦y≦3.9; 1.9≦z≦3.9; and the compound has anolivine structure or a NASICON structure, or the compound isisostructural with the LiVP₂O₇ structure or the vanadium oxy-phosphateVO(PO₄) structure; provided that the compound is not Mg_(a)MnSiO₄ orMg_(0.5)Ti₂(PO₄)₃.
 2. The magnesium battery electrode of claim 1,wherein b′ is 0 the compound has a formula of Mg_(a)M_(b)(XO_(z))_(y).3. The magnesium battery electrode of claim 1, wherein 0<a≦2, b is about1, y is about 1, and 3≦z≦3.9.
 4. The magnesium battery electrode ofclaim 1, wherein 0<a≦3, b is about 2, y is about 3, and 3≦z≦3.9.
 5. Themagnesium battery electrode of claim 1, wherein 0<a≦1.53, b is about0.5, y is about 1, and 3≦z≦3.9.
 6. The magnesium battery electrode ofclaim 1, wherein the electroactive compound has a unit cell atomicarrangement isostructural with a monoclinic or rhombohedral NASICON unitcell.
 7. The magnesium battery electrode of claim 1, wherein X is P. 8.The magnesium battery electrode of claim 1, wherein (XO_(z))_(y) is P₂O₇and the electroactive compound is isostructural with VP₂O₇.
 9. Themagnesium battery electrode of claim 1, wherein the electroactivecompound has a unit cell atomic arrangement isostructural with abeta-VOPO₄ unit cell.
 10. The magnesium battery electrode of claim 1,wherein the electroactive compound has a unit cell atomic arrangementisostructural with a cubic diphosphate TiP₂O₇ unit cell.
 11. Themagnesium battery electrode of claim 1, wherein the electroactivecompound has a magnesium diffusion barrier of less than 0.8 eV.
 12. Themagnesium battery electrode of claim 1, wherein the electroactivecompound is olivine MgFe₂(PO₄)₂.
 13. The magnesium battery electrode ofclaim 1, wherein the electroactive compound is NASICON MgFe₂(PO₄)₃ orMgV₂(PO₄)₃.
 14. The magnesium battery electrode of claim 1, wherein0.05≦b′≦3.9.
 15. The magnesium battery electrode of claim 1, wherein theelectroactive compound has an olivine structure.
 16. The magnesiumbattery electrode of claim 1, wherein the electroactive compound has aNASICON structure.
 17. The magnesium battery electrode of claim 1,wherein the electrode comprises an electrode layer comprising theelectroactive compound, said electrode layer disposed on the currentcollector.
 18. The magnesium battery electrode of claim 17, wherein theelectrode layer further comprises an electronically conductive additive.19. An energy-storing device comprising a first magnesium batteryelectrode of claim
 1. 20. The energy-storing device of claim 19, furthercomprising a second magnesium battery electrode selected from the groupconsisting of a magnesium battery electrode of claim 1, and a magnesiumbattery electrode comprising a magnesium metal or alloy as theelectroactive material.